This warning occurred because the printer is using a different paper size than the one you selected.
Using iOS 16? This is a bug and should be fixed soon
For many users, printing directly in the app has stopped working properly on iOS 16, especially iOS 16.4. If you are seeing this page, then you are probably affected. I am really sorry and I am working hard on updating the app with a fix. For now you can work around this by exporting your page as PDF or JPEG from the Print page, then by choosing Print in the list at the bottom of the export panel (or by following these steps). Make sure to select the correct paper size there.
Why did I get a blank print?
If the app had tried to print anyway, then your print could have been wrong, wasting your ink and paper. Instead you can reuse your blank sheet of paper and try again.
What can I do to fix this issue?
Choose a different paper size. If you tap on “Choose a Supported Paper Size”, the app will show you the paper sizes that are currently available in your printer.
If you have another way to print at the desired size (such as your printer manufacturer’s own app or a computer), you can export the page and print it outside the app, as explained here.
If the app is wrong and your paper size is supported, please check that you are using the latest Print to Size update (version 1.9).
It could also be a problem in your printer’s paper detection system. Please try turning off your printer and your iPhone or iPad, then turning them back on. It may help to open and close your paper trays or to refill them, in case your printer is failing to detect paper.